1. Field of the Invention
This invention relates to a clock generation circuit and, more particularly, a multi-phase, phase-locked loop (xe2x80x9cPLLxe2x80x9d) circuit used for synchronizing an electronic subsystem. The multi-phase PLL circuit can generate a first set of phase outputs interleaved with a second set of phase outputs, one or more of which can be forwarded to an electronic subsystem for synchronization, for example, in a clock and data recovery operation. The circuit preferably includes an oscillator within a first phase-locked loop and a multiple phase delay circuit within a second phase-locked loop, wherein the second phase locked loop induces a linear delay imparted to one or more delay circuits of the second phase-locked loop in order to interleave the phase outputs of the delay circuits with the first set of phase outputs.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Modern high-speed data communication systems typically use internal clock-referenced circuitry. The circuitry is designed to synchronize with, for example, an incoming data stream or reference signal. In most instances, a PLL circuit derives a sampling frequency from locking to the incoming data stream and generating the necessary clocking signals to which the receive circuitry is synchronized. Clock recovery circuits can employ a multi-phase PLL architecture, where the multiple phases"" aid in sampling, and in some cases over-sampling a transmitted data stream. By establishing phase coherence between the PLL based clocks and the data, information can be extracted through this synchronous detection method.
In its most basic form, a PLL consists of a phase/frequency detector, a filter, control circuitry, and a variable oscillator. The signal generated by the oscillator is continuously compared against an incoming reference clock signal. The reference clock signal preferably transitions from edges of, for example, the incoming data stream. Once compared, the control circuitry adjusts the oscillator output frequency so that the incoming data stream and the oscillator output are transitioning at the same frequency and ideally in phase with one another. Thus, PLLs can be used for the synchronization and re-timing of transmission input data in the form of clock signals.
Clock recovery circuits employing PLLs often benefit by producing a multi-phase oscillator output. For example, a single-phase oscillator output may transition at the same frequency and phase as the incoming data stream. If, however, the clock recovery is designed to receive higher data-rate frequencies, it can be advantageous to design the receiver architecture using a parallel circuit design approach that permits the majority of the receiver to operate at lower frequencies. The oscillator in the receiver can generate multiple phases which are separated by a fixed phase angle such that if two phases are separated by a fixed phase angle, the data receiver circuits can be clocked at a higher frequency to at least match that of the incoming data stream. Thus, PLLs used for data transceivers in clock recovery applications can benefit from using multi-phase clocks to effectively increase the sampling or synchronization rate of higher input frequency events with lower speed clock signals.
If the oscillator of the PLL can generate multiple phases, a parallel receiver architecture can be designed whereby a higher frequency incoming data stream can be clocked and sampled by a substantially lower frequency sampling clock using multiple phases whereby the effective sampling rate can be very high. This scheme can thereby permit the phase detector to more accurately track the higher bit rate of the incoming data. SONET bit streams may have a bit rate as high as 10 bit/s or even 40 Gbit/s (e.g., SONET/SDH standard OC-192 specifies a transmission rate of 9953.28 Mbit/s, and OC-768 specifies a transmission rate of 39813.12 Mbit/s). Consequently, the PLL and other components of the clock recovery circuit design strike a trade off between a parallel multi-phase architecture operating at lower speed compared to a full-rate, higher-speed design, if in fact the higher-speed single phase output design is even possible.
Unfortunately, it is not always a simple matter to produce multiple phase outputs from an oscillator, especially when the incoming data stream has bit rates in the Gbit/s range. As the number of phase outputs increase, the outputs from each inverter within a long chain of inverters used by an oscillator will have extremely small delay tolerance. For example, if a 45 xc2x0 out-of-phase condition is desired (i.e., an 8-phase oscillator output is needed), then even the slightest process variation used in forming the oscillator will negatively effect the oscillator""s ability to produce regularly spaced, 8-phase outputs separated ideally at 45 xc2x0. The problem is compounded as the number of phase outputs or oscillator frequency increases, thereby causing a conflict between the oscillator speed (or tap-to-tap delay) and the number of phase outputs. Conventional tap-to-tap delay bandwidth necessary to propagate a high-speed signal becomes semiconductor fabrication process limited. Moreover, tap-to-tap delay interpolation using analog techniques proves unreliable at high oscillation frequencies over full semiconductor fabrication process corners.
It would be desirable to introduce a clock generation architecture that can use multi-phase outputs from an oscillator without the aforesaid drawbacks. The desired clock generation circuit can receive a high-speed incoming data stream in excess of several Gbit/s to, for example, 10 Gbit/s. Moreover, the desired clock generation architecture should be designed to utilize an oscillator in a PLL that produces, for example, one-half of the multi-phase outputs, while another portion of the PLL circuit produces the other half of the multi-phase outputs. In this fashion, the tap-to-tap delay within the oscillator can be kept fairly large thereby not unduly taxing the semiconductor fabrication process corners when the oscillator is called upon to operate in the GHz range. In addition to its high speed, high density phase output capability, the desired PLL circuit should beneficially interleave sets of delay outputs, each operating at the same frequency and time delay amount across all process variations. Accordingly, the clock generation circuit, the PLL (and associated oscillator), and the control circuits of the desired electronic subsystem should be formed on the same monolithic substrate using conventional semiconductor fabrication processing techniquesxe2x80x94even when called upon to produce a large number of phase outputs and operate at significantly high frequencies.
The problems outlined above are in large part solved by an improved clock generation circuit and, more particularly, to a PLL architecture that can find application in a clock and data recovery circuit. The PLL circuit includes an oscillator having a first set of delay circuits that can be coupled in a ring topology. The PLL circuit can also include a second set of delay circuits separate and apart from the oscillator. The phase outputs from the second set of delay circuits are delayed with respect to phase outputs from the first set of delay circuits in order for phase outputs from the first and second sets of delay circuits to be interleaved with one another. Accordingly, the combined number of first and second sets of delay circuits produce the requisite number of phase outputs from the clock generation circuit. However, only approximately one-half of the total number of phase outputs can occur from the first set of delay circuitsxe2x80x94i.e., the oscillator. If the oscillator is called upon to operate at substantially higher frequencies, then the tap-to-tap delay can be kept fairly large, yet a high density, multi-phase output can be achieved from the combined first and second sets of delay circuits. In other words, instead of, for example, the oscillator producing 12 phase outputs at a tap delay of 30 xc2x0, the oscillator need only produce six phase outputs at a tap delay of 60. Minor variations in the overall tap delay will have a lessened effect if, for example, the tap delay within the oscillator was 60 xc2x0 rather than 30 xc2x0. Thus, process variations can be more easily tolerated.
As part of the clock generation circuit and overall PLL architecture, and attributable to the oscillator, is a first PLL adapted to produce a pair of voltages input to the oscillator for setting a frequency of the oscillator as well as the frequency of each output from the second set of delay circuits. In addition to the first PLL, a second PLL is attributable to the PLL architecture. The second PLL can produce another pair of control input signals to a linear delay network. The delay network is controlled by the second PLL to delay the phase outputs from the second set of delay circuits based on a difference in the pair of control values output from the second PLL. Preferably, the amount by which the linear delay network delays the phase outputs from the second set of delay circuits is a fixed percentage of the frequency of the oscillator. Also, the amount of delay is preferably x. 5, where x equals an integer value. More preferably, each phase output from the second set of delay circuits is equally spaced in phase relationship between corresponding pairs of phase outputs from the first set of delay circuits. In this fashion, the multi-phase outputs from the entire clock generation circuit are equally spaced. However, note that the amount of phase outputs from the second set of delay circuits that are interleaved with respect to the phase outputs from the first set of delay circuits can be greater than one. For example, the amount of incremental delay mentioned previously above can be xe2x80x9cx.333xe2x80x9d whereby now there will be two extra phases interleaved with respect to every two adjacent phase outputs from the first set of delay circuits. This aspect of the PLL architecture is limited by only the complete design constraints needed in a total clock recovery design.
In order to delay the phase outputs from the second set of delay circuits, a three-input phase detector is used. The three-input phase detector forms a part of the second PLL and provides the second pair of control inputs that vary the amount of delay within the second set of delay circuits relative to the first set of delay circuits. The amount of delay can be controlled by virtue of the relationship of pulse edges and, more particularly, the relationship between a pulse edge from a phase output of the second set of delay circuits and pulse edges from phase outputs of two of the first set of delay circuits. In this manner, the second PLL senses, through a feedback connection, phase outputs (or edges of phase outputs) not only within the second set of delay circuits attributable to the second PLL, but also the first set of delay circuits attributable to the first PLL. Thus, the first and second PLLs are nested, with the three-input phase detector sensing and thereby controlling, a phase output from the second set of delay circuits to be preferably in the middle, between phases, of a pair of phase outputs from the first set of delay circuits.
According to one embodiment, a clock generation circuit is provided. The clock generation circuit uses a PLL architecture that includes an oscillator having a first set of phase outputs. The PLL architecture also includes a set of delay circuits having a second set of phase outputs separate from, yet interleaved with, the first set of phase outputs.
According to yet another embodiment, the clock generation circuit includes a first set of delay circuits coupled in a ring and adapted to receive a first pair of differential voltages for setting an oscillation frequency of the ring. The clock generation circuit also includes a phase-locked loop adapted to produce a second pair of differential voltages for incrementally delaying phase outputs from a second set of delay circuits interspersed within and relative to the first set of delay circuits.
According to yet a further embodiment, a method is provided. The method can produce a plurality of phases by producing a first set of phase outputs spaced by a first phase amount. The method also includes delaying a second set of phase outputs approximately one-half the first phase amount and interspersing each of the second set of phase outputs between corresponding pairs of the first set of phase outputs. The combination of producing a first set of phase outputs, delaying, and thereafter producing a second set of phase outputs thereby forms the multi-phase output.
According to still another embodiment, a first PLL and a second PLL are provided. The first PLL sets the oscillation for each of the multi-phase outputs (since the second phase outputs are synchronized and delayed from the first phase outputs). The first PLL employs a feedback loop local only to the first phase outputs. The second PLL synchronizes a phase relationship (or delay) of the second phase outputs relative to the first phase outputs. The second PLL employs a feedback local not only limited to the second phase outputs but also using the first phase outputs. Thus, a phase relationship can be established between the signals in the core oscillating ring (designated as the first PLL) and the delay cell circuits in the second PLL loop.